1. Field of the Invention
This invention relates to a pneumatic tire improving a drainage performance without sacrificing the other tire performances such as a steering stability and the like, and more particularly to a so-called high-performance tire having a small aspect ratio and a wide tread width.
2. Description of Related Art
In the conventional pneumatic tire, for example, the tire for use in a passenger car, as shown in FIG. 5, it is general to have a tread pattern defined by arranging circumferential grooves 101-103 extending in parallel to an equatorial plane of the tire, and a plurality of slant grooves 104-106 opening to the circumferential groove 101, 102, extending from the circumferential groove 101, 102 toward each end of the tread, and obliquely extending to the equatorial plane of the tire.
In such a tread pattern, an excellent drainage performance can be given to the tire by mainly performing the drainage toward the longitudinal direction of the tire through the circumferential grooves and performing the drainage toward the lateral direction of the tire through the slant grooves.
In the tire having such a tread pattern, as means for further improving the drainage performance, it is useful to make larger a ratio of groove area or so-called negative ratio by enlarging the groove width and the like, or to arrange the slant grooves at a small angle with respect to the circumferential direction of the tire or form so-called high-angle grooves.
However, the increase of the negative ratio becomes lacking in the rigidity of the land in the tread portion, and also when the slant grooves are arranged as the high-angle groove, a corner part of the land defined by the slant grooves becomes acute and hence the rigidity of each land is lacking. In any case, the sufficient steering stability can not be obtained.
Moreover, as a construction of the tread pattern for balancedly satisfying both the drainage performance and the steering stability, it is useful to combine the circumferential grooves with the slant grooves as a basic construction and also to change the extending shape of the slant groove between a central region and a side region of the tread portion. Concretely, a portion of the slant groove located in the central region is formed as a steeply slant groove portion inclined at a small angle with respect to the circumferential direction of the tire and a portion of the slant groove located in the side region is formed as a gently slant groove portion inclined at a large angle with respect to the circumferential direction of the tire.
With respect to the tire having such a tread pattern that the drainage performance and the steering stability are balancedly satisfied, the inventor has made examinations for more improving the drainage performance and confirmed that when water existing in the central region of the tread in the tire contacting with ground is particularly taken into the slant groove and discharged therefrom toward the lateral direction of the tire, the water taken into the slant groove collides with a groove wall of a curved groove portion passing during the movement from the steeply slant groove portion to the gently slant groove portion to thereby easily cause disturbance of the water flow and such a disturbance largely lowers the drainage ability of the slant groove toward the lateral direction of the tire.